1. Field of the Invention
The present invention relates to a paper size detecting mechanism for detecting the size of paper (sheets of paper) stacked in a paper feed cassette unloadably loaded in an image forming apparatus, and to an image forming apparatus provided with such a paper size detecting mechanism. More particularly, the present invention relates to a paper size detecting mechanism that has a simple structure, that requires a reduced installation space, and that can prevent erroneous detection.
2. Description of Related Art
Image forming apparatus such as copiers, printers, facsimile machines, and the like are unloadably loaded with paper feed cassettes in which paper of different sizes can be appropriately stacked. An image forming apparatus loaded with such paper feed cassettes is designed to be capable of automatically detecting the size of the stacked paper.
In one typical example of a known paper size detecting mechanism, there is provided a switch that detects the position of a pressing member that, when paper is stacked in a paper feed cassette, is pressed against an edge of the paper to detect its position. Inconveniently, however, this detection mechanism requires as many switches as there are different sizes of paper and thus requires an increased number of components. Moreover, providing many switches requires an accordingly large space. This hampers the miniaturization of image forming apparatus.
As a solution to the above problem, there is known, for example, a paper size detecting mechanism that uses less switches and instead detects the paper size based on the combination of the on and off states of those switches. This improved paper size detecting mechanism is provided with—in the cabinet of an image forming apparatus—three push switches and—on a paper feed cassette—a segment gear which rotates in concert with a pressing member, and has cogs formed at its circumferential edge; an idle gear which is rotated by the segment gear; and a partly cut member which has a rack meshing with the idle gear, moves as the segment gear rotates, and has a recess and a protrusion that, when the paper feed cassette is loaded in the image forming apparatus, selectively press the switches to turn them on or off.
Inconveniently, however, the above paper size detecting mechanism requires gears, such as a segment gear, an idle gear, and a rack, and in addition a plurality of (three) independent switches. Thus, the mechanism is complicated and costly.
In another known paper size detecting mechanism, there are provided—on a paper feed cassette—a movable pressing member which is pressed against stacked paper; and a following member which swings in concert with the pressing member, and has one or more protrusions in a tip-end portion thereof in the loading direction of the paper feed cassette, and—in an image forming apparatus—a detecting member which has a detecting plate that elastically deforms when any of the protrusions on the following member makes contact with it; and a multiple switch which is operated by the action of the detecting plate. With this design, the use of a single multiple switch, which is compact and inexpensive, instead of a plurality of independent switches and the provision of a detecting member of an elastic material between the paper feed cassette and the switch helps simplify the mechanism and thus helps achieve space saving and cost reduction.
Inconveniently, however, with the above design, the detecting member is a plate-shaped member made of resin, and the elasticity of the resin is exploited to restore the plate-shaped member from the position (on position) where it presses a contact of the multiple switch to the position (off position) where it ceases to do so.
Thus, depending on the degree and frequency of deformation, the detecting member may go beyond the limit within which it can restore from elastic deformation and undergo creep deformation (plastic deformation). The detecting member then fails to completely restore from the on position to the off position and causes erroneous detection.